I recently realized that for as long as I've been playing 3.x, across all the builds and characters, I've never actually seen someone play a Mystic Theurge in one of our groups. The idea caught hold, and I began looking into how a MT could be built in Pathfinder.
In 3.5, Precocious Apprentice and Practiced Spellcasting made MT a fairly viable option, so naturally I wondered what PF might offer instead.
Early MT threads linked to the now-errata'd FAQ that allowed SLA's to qualify as spellcasting for PrC requirements, but newer threads instead pointed to Inner Sea Magic and the Spellcasting Guild rules, which grant a character Eclectic and Esoteric training. It also seemed pretty well agreed that while eclectic training was a nice little boost, Esoteric Training was off-the-charts - except for Mystic Theurges, who really needed it.
However, despite being a bit of an optimizer, the thought of me being able to get these two perks (on top of other nice benefits) for a combined total of something like 500-100g seems terribly imbalanced: a common suggestion was to make them feats (and I agree that two feats for an already feat-starved Mystic Theurge is a good price for the spellcasting benefit.)
So, with that in mind, I went about converting these perks into feats, also following-up with the idea that esoteric training should really only be available to the Mystic Theurge (and so limited it by making the Combined Spells class feature a prerequisite.)
I'd love some feedback before proposing these feats to our group.
Eclectic Training
Prerequisites: Ability to cast spells, Knowledge (any) 3 ranks, Spellcraft 3 ranks
Benefit: Choose one spellcasting class you have at least one level in: you increase your effective caster level in that class (including the number of spells you know and can cast per day) by one, to a maximum caster level equal to your total Hit Dice.
Esoteric Training
Prerequisites: Combined Spells class feature, Eclectic Training, Knowledge (any) 7 ranks, Spellcraft 7 ranks
Benefit: The bonus to caster level you gain from Eclectic Training increases to three (but is still limited by your total Hit Dice).
Select a second spellcasting class you have at least one level in: you increase your effective caster level in that class (including the number of spells you know and can cast per day) by one, to a maximum caster level equal to your total Hit Dice.